1. Field of the Invention
The present invention relates to a polytrimethylene terephthalate resin. More particularly, the present invention is concerned with a polytrimethylene terephthalate resin comprised mainly of trimethylene terephthalate recurring units, which has the following characteristics: an intrinsic viscosity [η] of from 0.6 to 4 dl/g; a molecular weight distribution (Mw/Mn) of from 2 to 2.7; a cyclic dimer content of not greater than 2% by weight; and a psychometric lightness L-value of from 70 to 100 and a psychometric chroma b*-value of from −5 to 25. By using the polytrimethylene terephthalate resin of the present invention, it becomes possible to produce an excellent shaped article stably on a commercial scale. Specifically, the shaped article produced using the polytrimethylene terephthalate resin of the present invention has high strength and excellent color. Further, the shaped article is free from the bleeding of the cyclic dimer to the surface of the shaped article, so that the shaped article is suitable for coating with a coating composition or adhesive agent and exhibits excellent adhesion property. The present invention is also concerned with a method for stably producing the polytrimethylene terephthalate resin with high productivity on a commercial scale.
2. Prior Art
A polytrimethylene terephthalate (hereinafter, frequently referred to as “PTT”) not only has characteristics similar to those of a nylon (e.g., soft feeling, excellent elastic recovery and good dyeability), but also has characteristics similar to those of a polyethylene terephthalate (hereinafter, referred to as “PET”) (e.g., wash and wear property, dimensional stability and discoloration resistance). Therefore, PTT has been attracting attention as an epoch-making material for a fiber. Further, due to the characteristics which a nylon does not possess, such as low hygroscopicity and discoloration resistance, and to the characteristics which a polybutylene terephthalate (hereinafter, referred to as “PBT”) does not possess, such as moldability, PTT can be used advantageously as an excellent raw material for producing shaped articles.
For further expanding the application fields of PTT, it has been desired to improve the strength and color of the fibers and shaped articles of PTT. For improving the strength of the fibers and shaped articles of a polymer, it is necessary to increase the polymerization degree of the polymer, and to narrow the molecular weight distribution of the polymer so as to reduce the amount of low molecular weight components in the polymer. Further, for improving the color of the fibers and shaped articles of a polymer, it is necessary not only to improve the whiteness of the polymer, but also to improve the heat resistance of the polymer so as to prevent the discoloration of the polymer, which is caused by the thermal history experienced by the polymer during the drying, melting and the like.
As a polymerization method for producing PTT, a melt polymerization process is widely known (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-262862 (corresponding to U.S. Pat. No. 5,340,909), WO98/23662, WO01/14450 and WO01/14451). In the above-mentioned documents, a method is described in which a melt polymerization is performed using a polymerization vessel equipped with a stirrer. The above-mentioned polymerization vessel has advantages in that it exhibits excellent volume efficiency and has a simple structure. Such a polymerization vessel can be used for efficiently performing a polymerization to produce a polymer having high polymerization degree on a small scale. However, when the above-mentioned polymerization vessel is used for performing a polymerization on a commercial scale, the depth of the liquid reaction mixture in the polymerization vessel becomes inevitably large, leading to a marked occurrence of heat decomposition of the polymer. Thus, a polymer having high polymerization degree cannot be produced on a commercial scale.
Various techniques have been proposed for producing a PTT having high polymerization degree by melt polymerization. Examples of such techniques include a technique in which a lower alcohol diester of terephthalic acid and trimethylene glycol are subjected to a transesterification reaction and a polycondensation reaction in the presence of a titanium compound, wherein the molar ratio of the lower alcohol diester of terephthalic acid to the trimethylene glycol is in the range of from 1/1.2 to 1/1.8 (Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-140992); a technique in which an organometal catalyst is used as a polycondensation catalyst, and an organic sulfonic acid or an aliphatic carboxylic acid is used as a cocatalyst (U.S. Pat. No. 4,611,049); a technique in which a tin catalyst is used as a polycondensation catalyst (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-262862 (corresponding to U.S. Pat. No. 5,340,909)); a technique in which a specific titanium catalyst is used as a polycondensation catalyst (Unexamined Japanese Patent Application Laid-Open Specification Nos. 2000-159875 and 2000-159876); a technique in which an antimony compound is used as a polycondensation catalyst (Chemical Fiber International Vol. 46, pp 263-264, 1996); a technique in which heat decomposition of PTT is suppressed by using a hindered phenol-type stabilizer having a specific structure (Unexamined Japanese Patent Application Laid-Open Specification No. Sho 51-142097); a technique in which the by-production of acrolein (formed by heating of a prepolymer and a polymer in air during the polymerization) is suppressed by blocking the terminals of the prepolymer and the polymer with a phosphorus-containing stabilizer and a hindered phenol-type stabilizer (WO98/23662 and WO99/11709); and a technique in which a specific titanium compound is reacted with a specific phosphorus compound in their respective amounts such that the phosphorus/titanium atomic ratio is in the range of from 1/1 to 3/1, and the resultant product is used as a polycondensation catalyst (Unexamined Japanese Patent Application Laid-Open Specification No. 2001-278971). However, the above-mentioned techniques are disadvantageous in that the molecular weight of the obtained PTT is not satisfactorily high, that a lowering of the molecular weight of the PTT occurs during the molding thereof, and/or that a discoloration of the PTT occurs. Thus, by the above-mentioned techniques, a PTT having satisfactory properties cannot be obtained. For example, when a PTT is produced by the technique described in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 2001-278971, the produced PTT has improved color; however, the polymerization rate becomes disadvantageously low, so that, especially when a PTT is produced on a commercial scale where the depth of the liquid reaction mixture in the polymerization vessel becomes inevitably large, heat decomposition of the polymer markedly occurs and, hence, it is substantially impossible to produce a PTT having high polymerization degree. Further, the technique described in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 5-262862 has the following problems. For improving the color of the PTT by this technique, it is necessary not only to control the amount of a tin catalyst so as not to exceed 525 ppm, based on the weight of dimethyl terephthalate, but also to use Hostaperm pigment (trade name), cobalt and the like. In addition, with respect to the improvement of the polymerization degree, a PTT having a high polymerization degree can be produced on a small scale where the amount of the reaction mixture in a polymerization vessel is only about 1 kg so that the depth of the reaction mixture in the polymerization vessel is small; however, it is difficult to produce a PTT having high polymerization degree on a commercial scale where the depth of the reaction mixture becomes inevitably large. Furthermore, as the above-mentioned tin catalyst, this patent document uses an organotin compound having a C—Sn bond, such as butylstannoic acid. Such an organotin compound is poisonous and, hence, is not a preferable catalyst.
A PTT produced by a conventional melt polymerization process has a problem in that the PTT contains a large amount of by-produced oligomers. Specifically, the conventional PTT contains oligomers in an amount of from 2.5 to 3.5% by weight, and about 90% by weight of the oligomers are a cyclic dimer which is a cyclic compound which is formed by a condensation of 2 terephthalic acid molecules. This cyclic dimer is disadvantageous in that it sublimates and is likely to bleed out from the PTT. Therefore, for example, when the conventional PTT is subjected to spinning, the cyclic dimer sublimates and is deposited around the spinning nozzle. The deposited cyclic dimer adheres to the resultant spun polymer (polymer fiber) which passes through the spinning nozzle, thereby causing the breakage or fuzzing of the polymer fiber.
Further, when the conventional PTT is subjected to injection molding, the cyclic dimer deposits on the inner surface of the mold (that is, a mold deposit occurs), so that the appearance and dimensional precision of the shaped article are spoiled. Further, the cyclic dimer bleeds out on the surface of the shaped article, thereby lowering not only the coating performance in the coating process using a coating composition or an adhesive agent, but also the adhesion property. Furthermore, during the production of a PTT by a conventional melt polymerization process, the cyclic dimer volatilizes from the polymer and deposits on the inner wall of the conduit provided in the production system used, thereby causing the clogging of the conduit.
The above-mentioned cyclic dimer is formed by the so-called “ring-linear chain equilibrium reaction” which occurs at the hydroxyl group-containing terminal portions of a PTT. Specifically, in a PTT resin, there is an equilibrium (i.e., a ring-linear chain equilibrium) between a cyclic dimer and a linear dimer unit (i.e., two consecutive trimethylene terephthalate molecules) at the hydroxyl group-containing terminal of a PTT, as shown in the following formula:
                wherein M represents a trimethylene terephthalate recurring unit and D represents a cyclic dimer.Therefore, even when the cyclic dimer is removed from the PTT by volatilization during the polymerization reaction, the cyclic dimer is immediately generated again in the same amount as that of the removed cyclic dimer, thereby causing the loss of the PTT. Thus, it was impossible to produce a PTT containing the unfavorable cyclic dimer in an amount reduced significantly. Thus, the yield of the PTT becomes inevitably low.        
It is known that a PET, which has a similar skeleton to that of a PTT, also contains oligomers. However, PET has an oligomer content as low as only about 1% by weight. Further, most of the oligomers in PET are cyclic trimers. The cyclic trimer of a PET is less likely to sublimate and to bleed out, as compared to the cyclic dimer of a PTT. Therefore, in the case of a PTT, the problems caused by the presence of oligomers are more serious than those in the case of a PET.
Further, as a method for producing a high molecular weight PTT which has an excellent heat stability, there is proposed a method for producing a PTT by a solid-phase polymerization process (in which prepolymer pellets are subjected to a polymerization) (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-311177, Japanese Patent Application prior-to-examination Publication (Tokuhyo) No. 2000-502392 and Korean Patent No. 1998-061618). In a solid-phase polymerization process, the polymerization is performed at low temperatures and, hence, the ring-linear chain equilibrium of the PTT can be displaced in the direction of the formation of the linear dimer unit (derived from the cyclic dimer) at the hydroxyl group-containing terminals of the PTT. For this reason, it has been reported that the solid-phase polymerization process is effective for reducing the cyclic dimer content of PTT.
However, as a result of the studies of the present inventors, it has been found that, even when a PTT having its cyclic dimer content reduced to less than 1% by weight is produced by the solid-phase polymerization process, the cyclic dimer is rapidly produced upon melting thereof during the melt-molding of the PTT, and the reduced cyclic dimer content of the PTT returns to the cyclic dimer content (about 2.5 to 3.5% by weight) of the prepolymer prior to the solid-phase polymerization in which the hydroxyl group-containing terminal portions are at the ring-linear chain equilibrium state. Therefore, when a PTT produced by the solid-phase polymerization process is used for producing a melt-molded product (e.g., a fiber, a film or an injection-molded product), the above-mentioned problems caused by the cyclic dimer cannot be avoided.
Further, the solid-phase polymerization is advanced by removing trimethylene glycol (hereinafter, referred to as “TMG”) from the surface of the PTT prepolymer pellets. Therefore, the polymerization degree varies depending on the size and shape of the pellets, and also varies depending on the position in the pellets. Therefore, the PTT obtained by this method is markedly non-uniform with respect to the polymerization degree (i.e., the PTT has a broad molecular weight distribution). Further, in the solid-phase polymerization, the solid prepolymer pellets get rubbed with one another over a long period of time, thereby generating polymer powder which becomes a loss. The presence of the polymer powder in the spinning process causes breakage or fuzzing of polymer fibers. For removing the polymer powder, an additional step therefor becomes necessary. Further, the solid-phase polymerization should be performed after the production of the prepolymer by the melt polymerization and the like, and thus, the entire process for producing a PTT becomes complicated and costly.
For solving the above-mentioned problems accompanying the solid-phase polymerization process, there have been proposed an improved melt polymerization process for producing a PTT having a high polymerization degree, wherein a disc ring or cage type reactor (WO00/64962) or a disc and donut conductor (U.S. Pat. No. 5,599,900) is used for efficiently removing TMG from the polymerization reaction system. However, each of the above-mentioned apparatuses is a vertical agitation-type polymerizer which is equipped with a rotary driving part. Therefore, in the above-mentioned process, when a polymerization is performed under high vacuum for obtaining a polymer having a high polymerization degree, it is impossible to seal the driving part completely. Thus, it is impossible to prevent the entrance of a trace amount of oxygen into the polymer and, hence, a discoloration of the polymer inevitably occurs. Especially, in the case of a PTT, such discoloration markedly occurs. When the driving part is sealed with a sealing liquid, it is likely that the sealing liquid gets mixed with the polymer, thereby lowering the quality of the resultant PTT. Further, even when the driving part of the apparatus is tightly sealed at the start of the operation thereof, the tightness of the sealing is lowered during the operation conducted for a long period of time. Thus, the above-mentioned process also has a serious problem with respect to the maintenance of the apparatuses.
On the other hand, a method for producing a resin (other than a PTT) is known in which the polymerization apparatus used therein does not have a rotary driving part, and a polymerization is performed by allowing a prepolymer to fall from a perforated plate (free-fall polymerization method). For example, a method is disclosed in which a polyester prepolymer is allowed to fall in the form of fibers in vacuo in an attempt to obtain a polyester having a desired molecular weight (U.S. Pat. No. 3,110,547). In this method, a polymerization reaction is performed in a one pass mode without recycling the polymer, because the recycling of a polymer which has already been allowed to fall in the form of fibers causes the lowering of the quality of the final polyester. However, the above-mentioned method has the following disadvantages. The polymer in the form of fibers is easily broken during the polymerization reaction, thereby causing a disadvantageously large fluctuation in quality of the final condensation polymer products. In addition, a low molecular weight condensation polymer is scattered from the polymer fibers during the polymerization reaction to stain the lower surface of the perforated plate. Due to such staining of the lower surface of the perforated plate, it becomes difficult to cause the polymer to fall in the form of fibers, so that the polymer fibers contact with one another to cause breakage of the polymer fibers or the polymer fibers are combined together to form a thick fiber in which the reaction does not proceed efficiently.
In order to solve these problems, various methods have been proposed. Examples of such methods include a method in which a polyester or a polyamide is produced by allowing a prepolymer to fall along and in contact with the surface of a perforated guide or a wire guide, which is vertically arranged in a reaction vessel, so that the polymerization of the prepolymer is effected during the fall thereof (Examined Japanese Patent Application Publication No. Sho 48-8355 and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 53-17569); a method for continuously condensation-polymerizing bis-(β-hydroxyethyl) terephthalate (which is an initial-stage condensation product of polyethylene terephthalate (PET)), in which bis-(β-hydroxyethyl) terephthalate is allowed to fall along and in contact with wire guides in an atmosphere of inert gas, wherein the wire guides are hung vertically from the holes of a perforated plate, so that the polymerization of bis-(β-hydroxyethyl) terephthalate is effected during the fall thereof (Examined Japanese Patent Application Publication No. Hei 4-58806); and a method for producing a melt-polycondensation polymer (such as a polyester, a polyamide or a polycarbonate) in the form of a film, in which a melt-polycondensation prepolymer is caused to absorb an inert gas, and then, polymerized under reduced pressure (WO99/65970 which also discloses an apparatus used in the method).
However, each of the above patent documents only describes a method for producing a polyester (such as a PET) or a nylon, and has no proposal or suggestion about the production of a PTT. As a result of the studies of the present inventors, it has been found that, when any of the above-mentioned methods are simply applied to the production of a PTT (that is, when the production of PTT is conducted by any of the above-mentioned methods, using raw materials and conditions which are conventionally used in the production of a PTT), a foaming of a polymer vigorously occurs, thereby staining the lower surface of the perforated plate or the inner wall of the reaction vessel having the guides provided therein. The PTT is susceptible to heat decomposition, as compared to, for example, the PBT. Therefore, the stain caused by the above-mentioned vigorous foaming of the polymer is easily decomposed. When the resultant decomposition products get mixed with the polymer, disadvantages are caused that the quality of the polymer is lowered, that the desired polymerization degree cannot be obtained, and that the obtained PTT suffers discoloration. Thus, the simple application of the above-mentioned methods to the production of PTT is accompanied by a problem that it is difficult to achieve a satisfactorily high polymerization degree. In addition, the final PTT contains low molecular weight polymers, which result in a broad molecular weight distribution of the final polymer and are likely to lower the mechanical strength of an ultimate shaped article.
In recent years, there has been proposed a technique in which a PTT prepolymer having a specific polymerization degree is caused to flow through the holes of a perforated plate, and then allowed to fall along and in contact with a guide at a specific temperature under reduced pressure to thereby perform a polymerization of the PTT prepolymer during the fall thereof (Japanese Patent Application No. 2002-172735). By this technique, it has, for the first time, become possible to produce a PTT having a high polymerization degree and an excellent color. However, for meeting the recent demand for high quality fibers and shaped articles, it has been desired to further improve the color and mechanical properties of the polymer. Further, a PTT produced by the conventional melt polymerization process had a disadvantageously high cyclic dimer content and, hence, it has been desired to reduce the cyclic dimer content. Furthermore, it has been desired to solve a problem which arises when the continuous production of PTT is conducted by any of the conventional techniques, that is, a problem that the cyclic dimer which volatilizes from the polymer during the polymerization is deposited on the inner wall of the conduit provided in the production system, thereby causing the clogging of the conduits, so that a stable production of PTT becomes difficult.